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Leak Testing with Nitrogen

Other Methods of Leak Detection

The subject of leak detection
would not be complete without discussing the benefits of nitrogen
testing. Below you will find excerpts from an interesting article which
first appeared in Cool Profit$ Magazine. This article was written by John Noble, and has been reprinted here with permission from Cool Profits Magazine.

Overcoming Residue Surface Tension: Leaks are harder to find because
leak testing is performed with the system turned off. Lost are the benefits
of an operating system: a) constant washing of interior surfaces of
components; b) higher operating pressures that encourage leaks. With a
system at rest, the undisturbed residues mentioned above are able to coat
the insides of the evaporator, condenser, compressor, and other
components. When Residue Surface Tension is greater than the interior
pressures’ ability to displace it, there is no leak occurrence, thus no leak
detection.

Residue Displacement: If surface tension is the culprit, then how do we
overcome it? Answer: By adding 4-ounces of chlorine-based refrigerant
R-22 to the system. R-22 disturbs the surface tension. We follow that by
pressurizing the system with 175-200 psi of nitrogen. Both are cheap.

At the higher system pressure, the R-22 overcomes the residue surface
tension and forces the leak path to reopen. Now, enough R-22 gas is
available so that leaks are detected easily. The R-22 is the
residue-displacing agent.

R-22 summary. R-22: (a) creates a leak path because of its oil solubility
and residue displacement potential; (b) is more readily detectable by
electronic leak detectors than HFCs; (c) is non visual-dependent, unlike
dyes, thus can be used effectively to locate leaks in enclosed areas; (d) will
not cross-contaminate nor cause any harmful consequences to CFC, HFC,
nor blend refrigerant systems (i.e., after leak testing an HFC (R-134a)
system using R-22, there is typically zero percent cross-contamination if
evacuated afterwards).

Nitrogen summary. Nitrogen has the following qualities: (a) inert, very dry
and non-flammable; (b) does not go into solution with refrigeration oil to
create non condensable pressure problems (i.e., after leak testing an HFC
(R-134a) system using R-22 and nitrogen, there is typically zero percent
non condensables (nitrogen or air), if evacuated afterwards).

Diluted Liquid Soap. Speed is important to technicians when performing a
leak test. Large and medium size leaks can be quickly located in exposed
areas such as under hood components by applying diluted liquid dish washing soap directly to suspected leak points.

Multiple Leaks. If a large leak is discovered repair it and perform a
follow-up leak test. Leak tests should be performed following each repair
until all leak points are found and fixed.

Electronic Leak Detectors. Heated diode leak detectors are currently the
best non-visual, dependent leak detection instrument for sensing a
gaseous leak. They have the necessary characteristics of sensitivity,
repeatability and recoverability (after a leak), which are so important.

Releasing Test Mix. The EPA approved the releasing of the “test mix”
(R-22 & nitrogen) with the stipulation that: “All existing refrigerant within
the system be recovered properly, and a 102mm (about 4”) mercury vacuum
drawn on the system.”

By their definition, the R22-nitrogen test mix used for leak testing is not
considered a refrigerant and therefore may be released to atmosphere.
Their rationale is that one tiny loss which results in the discovery and
repair of a leak reduces the greater loss over the life of a system. It’s better
than multiple recharges of refrigerant and multiple losses to atmosphere.

Note: We’ve learned that the R22-nitrogen test mix should be released
outside the building via copper (or other) tubing to prevent fouling the air
inside the shop. This prevents false alarms by the leak detector. Also,
always maintain a well-ventilated work area.

Summary
There is currently no equally effective (in all situations) substitute for the
combination leak test method described above. It has neither the problems
nor shortcomings of dye. I have heard the argument that nitrogen is
dangerous. Well, the stationary industry has been using it for at least 60
years, and my company has been using it since 1973. Some of the major
stationary manufacturers think that is important enough to dedicate a
section of their training manuals exclusively to its use for leak testing their
a/c systems. It is not unsafe, unless one uses it improperly. Mobile a/c
technicians are simply untrained in its use. What a shame, it is so simple
and effective and easier, safer, and cheaper, etc.

Once upon a time, folks were afraid to fly in an airplane that didn’t have a
propeller—it was thought to be too dangerous. I encourage any a/c
technician to get a good quality nitrogen regulator and bottle of nitrogen
from a welding supply store. Set the regulator to 200 psi. It is safe to use
with all conventional a/c systems, and will not damage any system
components.

Make sure that the nitrogen bottle is secured to a wall or in a dolly, and
that everyone in the service department is taught not to fool with the
regulator setting. Or, install an Allen head set screw and lock nut with the
regulator so that it can’t be adjusted once it is pre-set. We have safely
tested a/c system components at 200-psi nitrogen pressures on over 40,000
tests. Remember, 200 psi is only about 50 psi higher than the compressed
air lines used by you and every other technician nation wide. I encourage
all technicians who aspire to become more professional in a/c service to
invest in a good quality nitrogen regulator and bottle. Then take the time to
practice the method. You will be doing a service to yourself and your
customers. $$$

About the Author: Before
opening an automobile repair business, Cool Flow, Inc. founder John
Noble spent 7 years in the aircraft service industry, and prior to 1971
worked as an FAA certified airframe & power plant technician.
During the next 4 years he began to make the transition to specializing
in automobile a/c related services. Today, the company is currently
undergoing another transition into development of several a/c related
products.